1. Field of the Invention
This invention relates to oxa-alkane polyphosphonic acids and salts thereof, to their use as thresholders, and to complexing compositions containing these oxa-alkane polyphosphonic acid derivatives.
2. Statement of Related Art
It is known from published German application no. 34 34 667 that 4-dimethylamino-1-hydroxybutane-1,1-diphosphonic acid can be prepared by phosphonylation from 4-dimethylaminobutyric acid hydrochloride.
In addition, it is known from European Patent No. 39 033 that .omega.-amino-1-hydroxyalkylidene-1,1-bisphosphonic acids can be prepared by phosphonylation from .omega.-aminocarboxylic acids. Both products show complexing and threshold-active properties.
In view of the increasing pollution of waters by ethylenediamine tetraacetic acid (EDTA), the search for suitable substitutes has become an acute problem. The poor biodegradability of EDTA is a particularly troublesome disadvantage. Since, in contrast to most phosphonate complexing agents, EDTA does not show any threshold activity, it has to be used in stoichiometric quantities for adequate complexing in numerous industrial processes where scaling occurs under the effect of the hardness elements in water. In many cases, therefore, EDTA is replaced by the structural analog, ethylenediamine tetramethylene phosphonic acid (EDMP), which shows the desired threshold effect. However, like most other complexing and threshold-active phosphonates, EDMP is also attended by the disadvantage of poor biodegradability.